Ultra-high-molecular-weight polyethylene (UHMWPE) has a molecular weight that is 10 to 20 times greater than high-density polyethylene (HDPE). It has been defined by ASTM as having a weight average molecular weight (Mw) greater than 3,000,000.
In addition to the chemical resistance, lubricity, and excellent electrical properties of conventional HDPE, UHMWPE offers major advantages in toughness, abrasion resistance, and freedom from stress-cracking.
UHMWPE is produced by Ziegler polymerization. U.S. Pat. No. 5,756,600 teaches how to make an ultra-high-molecular-weight polyethylene with Ziegler catalyst. The process requires exceptionally pure ethylene and other raw materials. An .alpha.-olefin comonomer, such as 1-butene, may be incorporated into UHMWPE according to U.S. Pat. No. 5,756,600. Like conventional HDPE, UHMWPE made by Ziegler polymerization has a broad molecular weight distribution, and usually its polydispersity Mw/Mn (Mw: weight average molecular weight, Mn: number average molecular weight) is within the range of 5 to 20.
Newly developed metallocene and single-site catalysts advantageously provide polyethylene and other polyolefins with very narrow molecular weight distribution (Mw/Mn from 1 to 5). The narrow molecular weight distribution results in reduced low molecular weight species. These new catalysts also significantly enhance incorporation of long-chain .alpha.-olefin comonomers into polyethylene, and therefore reduce its density. Unfortunately, however, these catalysts produce polyethylene having a lower molecular weight than that made with Ziegler catalyst. It is extremely difficult to produce UHMWPE with metallocene and single-site catalysts. For example, U.S. Pat. No. 5,444,145 teaches preparation of polyethylene having a Mw up to 1,000,000 with a cyclopentadienyl metallocene catalyst. However, its molecular weight is significantly lower than the required for UHMWPE.
UHMWPE that has a narrow molecular weight distribution is unknown and is needed. A feasible process for preparing it would be valuable.